This application claims priority to an application entitled xe2x80x9cMethod For Controlling Channel Access by Access Slot Reserving in Mobile Communication System and Base Station Device Thereforxe2x80x9d filed in the Korean Industrial Property Office on Jun. 13, 1998 and assigned Serial No. 98-22218, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates generally to a communication system, and in particular to a channel access control device and method in a code division multiple access (CDMA) mobile communication system. Specifically, the present invention relates to a channel access where a base station reserves a specific access slot for a mobile station in the event that the mobile station has data to transmit which exceeds one radio frame in length to increase transmission efficiency and system performance.
2. Description of the Related Art
In a mobile communication system, random access to a base station by mobile stations may cause collisions because information about available channels is not exchanged among the mobile stations. As an alternative plan to reduce the probability of collision, an access slot concept has been introduced.
Mobile stations attempt, in sync with a base station, to transmit data according to an access offset in a unit of radio frame for a physical layer. One radio frame includes four offsets, and the collision occurs when two or more mobile stations access the same code with the same offset. Such an offset-unit transmission system is called a slotted ALOHA system, which is typically used in mobile communication system. The slotted ALOHA system allows the mobile stations to have a reduced delay time, and thus, provides a high channel utilization efficiency, as compared with a standard ALOHA system.
More specifically, a mobile station attempts to transmit data referring to an access code for random access and offset information, when there is a signaling message or packet data to transmit to the base station. When a message in a upper layer (i.e., Link Access Control Sublayer (LAC Sublayer) exceeds one frame in a length, the upper layer message is segmented in a unit of radio frame length and corresponding indication bits are added. Upon completion of constructing the radio frames, the mobile station transmits the radio frames to the base station via a specific channel at an offset start time and awaits receipt of an acknowledgement. Upon receipt of data from the mobile station, the base station performs error correction using a cyclic redundancy check (CRC) code to determine whether the received data has an error, and transmits the acknowledgement to the mobile station.
Further, when there are additional data to receive, the base station assembles received data after complete reception of the additional messages. After assembly of the messages, the base station transmits data to the upper layer. Upon receipt of an acknowledgement from the base station, the mobile station transmits additional data or new data after random delay. Upon failure to receive an acknowledgement within a preset time after data transmission, the mobile station retransmits the data.
FIG. 1 is a diagram illustrating a frame structure of a conventional Broadcast Control Channel. A Broadcast Control Channel (BCCH) includes a pair of radio frames, and a BI field indicates whether the frame is a first radio frame or a second radio frame. Further, a TX_PWR field denotes a transmission power of the Broadcast Control Channel; an SFN field denotes a system frame number, which increases by one for every radio frame; a UP_INTERFACE field represents a measured value for a latest reverse interference; a W field indicates whether data is continued in the next frame, in the case where data in a link access control (LAC) sublayer being a upper layer of the MAC layer is segmentally transmitted; a CRC field includes a cyclic redundancy code; and a TA field includes tail bits which are used for initialization of an orthogonal coder. In an IMT-2000 system, for assignment of a signaling channel, a mobile station sends a channel assignment request via a Random Access Channel (RACH), and a base station then sends an acknowledgement for the channel assignment request via a Forward Access Channel (FACH). The Broadcast Control Channel is used when the base station transmits system information to the mobile stations.
FIG. 2 is a diagram illustrating a frame structure of a Random Access Channel. In FIG. 2, a D field denotes a dummy bit; a U/C field indicates whether the frame data is user data or control data; a TN field represents whether the information is used between the base transceiver system (BTS) and the mobile station or between a base station controller (BSC) and the mobile station; an S field includes a sequence number, wherein when the frame has an error according to the CRC checking result, a corresponding sequence number is written in the S field to retransmit the data; and a PID field denotes a packet ID for identification of the mobile station. The Random Access Channel is a physical channel, which is used when the mobile station transmits to the base station, control data for requesting assignment of a dedicated channel and small user packet data without need to establish a dedicated channel.
FIG. 3 is a diagram illustrating a frame structure of a conventional Forward Access Channel. In FIG. 3, an NA (the number of ACKs) field denotes the number of ACKs for random access of mobile stations. The Forward Access Channel is a physical channel, which is used when the base station transmits to the mobile station control data or acknowledgement information about random access. As illustrated, a 64K-channel frame is divided into 4 sub-frames each having 16K transmission channels.
FIG. 4 is a diagram illustrating a channel access procedure via an RACH and a FACH in a conventional IMT-2000 system. In FIG. 4, xe2x80x9cACKxe2x80x9d represents a successful access to the base station by the mobile station; xe2x80x9cFACHxe2x80x9d denotes a Forward Access Channel; xe2x80x9cRACHxe2x80x9d denotes a Random Access Channel; and xe2x80x9cBCCHxe2x80x9d denotes a Broadcast Control Channel.
Referring to FIG. 4, the mobile station is synchronized with the base station. The mobile station then acquires an offset for an RACH to access, and attempts to transmit channel access information via the RACH. In the event that the channel access information transmitted via the RACH is received without collision, the base station notifies the mobile station by transmitting an acknowledgement via the FACH. Upon receipt of the acknowledgement, the mobile station attempts to access the channel again after a random delay. As illustrated, the mobile station transmits data at a 16th access slot after the random delay.
As described above, the slotted ALOHA system does not take into a consideration the event where there is consecutive data to transmit. Therefore, when consecutive data exceeding one radio frame in size is segmented in a unit of radio frame size and then transmitted in random access, transmission of the consecutive data and transmission of new data may be equally managed, thereby causing a transmission delay problem. That is, even for transmission of the consecutive data, a next frame is transmitted with the random delay after an acknowledgement is received for one radio frame. If the consecutive frame and the inconsecutive frame make a contention on an equal basis, transmission of the present frame cannot be guaranteed. In addition, when a packet data service is provided via a common signaling channel, a time delay occurs. That is, even for transmission of the consecutive data, a next frame is transmitted with the random delay after an acknowledgement is received for one radio frame. If the consecutive frame a collision problem, transmission of the upper layer message cannot be guaranteed to be transmitted in present duration. In addition, when a packet data service is provided via a Random Access Channel, long time delay occurs.
It is, therefore, an object of the present invention to provide a channel access control device and method in a mobile communication system where a base station reserves a specific access slot for a mobile station when the mobile station has data to transmit which exceeds one frame in length to thereby increase transmission efficiency and system performance.
To achieve the above object, there is provided a channel access method for a base station in a mobile communication system. The method includes the steps of (a) receiving channel access information from a mobile station via a reverse access channel; (b) examining a length oftransmission data, included in the received channel access information, to determine whether the transmission data continues in at least one succeeding frame; and (c) reserving a specific access slot on the reverse access channel for the mobile station via a forward access channel when the data continues in the at least one succeeding frame.
There is also provided a channel access method for a mobile station in a mobile communication system. The method includes the steps of (a) receiving access channel reserve information from a system broadcast channel and determining how many access slots on a random access channel are reserved for a next system broadcast channel data transmission duration; (b) receiving reverse confirm information via a forward access channel and determining whether an access slot is reserved for the mobile station for the next system broadcast channel data transmission duration; (c) upon detection of a reserved access channel, transmitting an access channel frame at the reserved access slot on the random access channel; (d) determining whether a response is received from a base station within a first predetermined time; and (e) upon receipt of a response within the first predetermined time, analyzing the response to determine whether an access slot is reserved, and returning to step (c) when an access slot is reserved.